Mr Hat-and-Cloak is Future Harry, and he's meddling with everything to prevent First Harry from going Dark.
I doubt it. If Harry simply told himself, he'd probably listen.
Dunno, depends on how far they've diverged. I know if I went back 10 years and told my past self not to do a certain thing I'd have even ignored myself, much as I ignored everyone else in my life that tried to warn me away back then. If Harry really wanted to do something he could conceivably (and possibly even correctly) reason that his future self has had such a radical shift in values that they are antagonists now, and his future self is yet one more obstacle to overcome.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: